The present invention relates to a power plant and, more particularly, to a steam power plant including an arrangement for retaining a vacuum in a condenser of the power plant during a short term outage or shutdown of the power plant.
In steam power plants, vacuum within the condensers thereof is not usually retained during long term outages or shutdown of the turbines of power plant; however, in short term outages or shutdowns, the steam may or may not be retained depending upon the particular operating circumstances. Both the retention and release of the vacuum have merits and disadvantages.
A disadvantage of retaining vacuum within the condensers of the steam power plant during a short term outage or shutdown of the steam turbines resides in the fact that additional power must be consumed during the outage or shutdown simply to retain the vacuum condition in the condensers, with a large portion of the additional power consumption representing a power loss necessitated by continuous operation of circulating water pumps in the steam power plant.
If, for example, the circulating pumps in the steam power plant are stopped during a short term outage or shutdown of the steam turbine in order to save unnecessary power consumption, the vacuum in an interior of the condensers is broken so that a troublesome restarting operation of the steam power plant is required, which restarting takes a considerably long period of time. Moreover, since condensate water within the condensers comes into contact with the atmosphere and absorbs oxygen, a quality of the condensate water is considerably lowered thereby increasing the rate of corrosion.
The power necessary to operate the circulating pumps to retain the vacuum in condensers of a steam power plant during a shutdown or outage is considerable. For example, with a thermal electric plant of 700 MW, the necessary power to operate the circulating pumps may represent an annual power rate of several million dollars.
In view of recent concerns regarding energy conservation, there has been an increase in the frequency of the shutdowns or outages in steam turbine plants employed for power generation as well as in other plants. More particularly, steam turbines with a combined cycle are subjected to start up and shutdown alternately at a high frequency so that the abovenoted disadvantages appear to a relatively large extent.
For the purpose of reducing power costs during an outage or shutdown, it has been proposed to operate the circulating pumps at about a 50% load to maintain a vacuum in the condenser by, for example, operating either one of two circulating pumps which are adapted to be operated in parallel.
However, a disadvantage of the last mentioned proposal resides in the fact that, since the flow rate of the condenser cooling water is reduced by about one half and, correspondingly, the speed of water is reduced by about one half, contaminants or pollutants such as, for example, microorganisms or marine biology from, for example, ocean cooling water, tend to adhere and collect on inner wall surfaces of the coolant pipes thereby adversely affecting the overall reliability of the entire power plant system and requiring more frequent time consuming cleaning operations of the coolant circulation system.